1. Field of the Invention
This invention relates to the field of power systems, and more particularly, to power systems that reduce switching losses.
2. Description of the Prior Art
The majority of power system's used in modern data processing and telecom equipment use multiple power conversion stages, for example switching power converters. The multiple stages can include a rectifier stage, front-end PFC boost converter stage, DC-DC output stage, and stand-by power stage. One important consideration in the design of such power systems is Power Factor Correction (PFC), a power conversion measure related to line-current harmonics. More specifically, PFC is a measure of how the voltage waveform follows the current waveform, a factor that can cause line-current harmonics.
FIG. 1 shows a conventional multi-stage power system 100. As shown, the system 100 includes a rectifier stage 102, a front-end PFC boost converter stage 104, a DC-DC output stage 106 and a stand-by power stage 108. The rectifier stage 102 includes a rectifier and a filter coupled to an input voltage source VIN to provide a rectified voltage to the front-end PFC boost converter stage 104. The front-end PFC boost converter stage 104 reduces the line-current harmonics in order to allow compliance with various worldwide specifications governing the harmonic limits, for example, as defined by IEC. The DC-DC output stage 106 is a dc-to-dc conversion stage for providing one or more output DC voltages VO1, VO2, . . . VOn. The stand-by power supply stage 108 provides housekeeping power and ensure system functionality when the system is in low-power (stand-by or sleep) mode and the main power is shut down to reduce power consumption.
A preferred topology at higher power levels is known as continuous-conduction-mode (CCM) boost converter stage. A CCM is a mode when the boost converter stage exhibits reverse recovery characteristics associated with its rectifier. In recent years, significant efforts have been made to improve the performance of high-power boost converters by addressing the adverse effects of reverse recovery. The majority of these development efforts have been focused on increasing power conversion efficiency and electromagnetic compatibility (EMC).
Soft-switching is one way of reducing reverse-recovery-related to switching losses and EMC problems. Switching loss in a switch is loss associated with the product of voltage and current through a switch at switching instance. Soft-switching is accomplished by controlling the switching instants during intervals when voltage or current through the switch are near zero. When the voltage at the switching instant is near zero, the soft-switching is called zero-voltage switching or ZVS. On the other hand, when the current at the switching instant is zero, the soft-switching is called zero-current switching or ZCS.
Conventional soft-switched boost converters use passive or active snubber circuits that control the rectifier switching rate. For example, FIG. 2 shows a boost converter stage disclosed in the Co-pending Application that uses an active snubber circuit. The shown soft-switched boost converter uses inductor LS to store energy for creating ZVS conditions for switch S. In FIG. 2, a transformer TR provides the reset of the current in Ls for creating ZCS conditions for switch S1.
The DC-DC Output stage 106 provides a regulated output voltage. This stage also provides isolation between the output and the preceding stages of the power system. Generally, the DC-DC output stage 106 can be implemented with any isolated converter topology, for example, single ended topology and multiple switch topology. Examples of these topologies include forward, flyback, push-pull, or bridge-type converter topologies. The topology selection is primarily based on considerations of output power level and often require balancing of performance vs. cost. Typically, for higher power levels (e.g., above 600 W), multiple switch topologies such as the two-switch forward, half-bridge, and full-bridge converters are employed. For lower power levels (e.g., below 300 W), single switch topologies such the flyback and forward converters are used. At medium power levels in the range from 300 W to 600 W, the two-switch forward topology is popular for its simplicity and relatively good performance. This is because a two-switch forward converter does not require separate transformer reset circuitry and the maximum stress on its primary-side switches is limited to the maximum input voltage. FIG. 3 shows the DC-DC Output Stage 106 implemented using a conventional two-switch forward converter.
FIG. 4 shows the stand-by power supply stage 108 implemented using the flyback topology, which has a low part count and the ability to operate efficiently in a wide input-voltage range. It is known to reduce the power consumption of the power supply 108 by turning off the boost converter front end stage 104. In some cost-critical applications, the stand-by flyback converter of FIG. 4 is implemented without using a controller, i.e., it is implemented as a self-oscillating converter.
There has been a need to decrease the size of power systems, including power conversion equipment. Advanced packaging and thermal management techniques have been used in power systems that operate at higher switching frequencies. A further size reduction has been achieved through component integration, for example, integrating semiconductor switches with drive, control and/or supervisory circuits and/or by integrating magnetic components such as transformers and inductors on the same core.
However, there still exists a need to further reduce the size of power systems beyond what is known in the art.